MICROFICHE APPENDIX
Not Applicable
The present invention relates to rotary fluid pressure devices such as low-speed, high-torque gerotor motors, and more particularly, to improved spool valve type gerotor motors.
Low-speed, high-torque gerotor motors are typically classified, in regard to their method of valving, as being "spool valve" motors or "disc valve" motors. As used herein, the term "spool valve" refers to a generally cylindrical valve member in which the valving action occurs between the cylindrical outer surface of the spool valve, and the adjacent internal cylindrical surface ("bore") of the surrounding housing. By way of contrast, the term "disc valve" refers to a valve member which is generally disc-shaped, and the valving action occurs between a transverse surface (perpendicular to the axis of rotation) of the disc valve and an adjacent transverse surface.
Although the present invention may be utilized with gerotor motors of various types of valve arrangements, it is especially suited for use with spool valve motors, and will be described in connection therewith. Furthermore, the invention is especially suited for use with a spool valve motor in which the spool valve is rotated by the main torque transmitting drive shaft, and will be described in connection therewith.
Also, although the present invention may be utilized with gerotor motors of various sizes and various flow and pressure ratings, it should be noted that the use of spool valves has typically been limited to smaller motors, having relatively lower flow and pressure ratings. This has been true partly because of the inherent limitations in spool valve motors wherein there is a radial clearance between the spool valve and the adjacent cylindrical surface or bore of the housing. This radial clearance provides a cross port leakage path which can be eliminated, but only with great difficulty, unlike in the case of disc valve motors, wherein the adjacent valving surfaces are biased into sealing engagement. However, it is becoming more typical for customers (e.g., vehicle manufacturers) to want to use spool valve motors in operating conditions of relatively low speed and relatively high torque. For example, the subject embodiment of the invention is now regularly being utilized, in development, at 5 to 10 rpm or less, and at pressure differentials of about 3000 psi., producing output torques in excess of 5000 lb.-in.
Among the performance characteristics which are considered quite important in low-speed, high-torque gerotor motors are volumetric efficiency and smooth operation, which are somewhat related to each other, Volumetric efficiency may be viewed as the ratio of the actual instantaneous speed of the motor (under certain flow and pressure conditions) to the theoretical instantaneous speed (under the same flow and pressure conditions. When the motor is being operated at a very low speed (low flow), and at a fairly high torque (high pressure), if there is a substantial amount of leakage, thus reducing the volumetric efficiency, the motor will probably run rough, i.e., the torque and speed will not remain consistent but will vary noticeably. Such inconsistency will typically result in rough operation of the associated piece of equipment, which is not acceptable to most customers or to the vehicle operators.
Another important performance characteristic of a gerotor motor is the mechanical efficiency, which may be viewed as the ratio of the actual output of the motor, in terms of torque, to the theoretical torque which should result from the pressure drop across the motor. As is well understood by those skilled in the art, friction is one of the main causes for loss of mechanical efficiency, for example, the frictional losses in the various spline connections, etc. Unfortunately, it is common in gerotor motors that whatever increases volumetric efficiency (e.g., closer clearances) reduces mechanical efficiency, and vice versa.
In many spool valve motor designs, the spool valve and the motor output shaft are formed integrally, with torque output of the gerotor gear set being transmitted to the output shaft by means of a dogbone drive shaft. At relatively low pressures, the various valve passages on the spool valve and in the housing achieve proper communication with each other, and the fluid is communicated to and from the gerotor gear set as intended. However, as the operating pressures rise, the torque being transmitted causes the dogbone shaft to "twist", a phenomenon which is generally understood by those skilled in the art. As the dogbone twists (perhaps as much as one or two degrees or more) under relatively high torque loads, the timing of the communication of each spool passage and its adjacent housing passage is no longer correct, relative to the then-current condition of its associated volume chamber in the gerotor gear set.
In other words, what is happening in the spool valving "lags" behind what is happening in the volume chambers of the gerotor gear set. By way of example only, as one of the volume chambers becomes a maximum volume transition chamber (which will be illustrated in greater detail subsequently), the spool valving will continue for one or two more degrees of rotation to communicate high pressure fluid into that volume chamber, the volume of which is not changing. The instantaneous result will be that the volume chamber has begun to shrink while still communicating with high pressure. Then the valving shuts off and the chamber shrinks further, and because of overlap in the valving, with no way to relieve pressure in the chamber, the fluid pressure will rise rapidly creating a pressure pulse or spike in that volume chamber. Such incorrect timing will result in a number of problems in the gerotor, each of which will have a further detrimental effect on volumetric efficiency and motor smoothness.